The invention relates to calibrators for rf receiving systems and specifically to a calibrator for a logarithmic amplifier.
Logarithmic amplifiers are frequently utilized in rf receiving systems in which it is important to determine input signal levels over a large dynamic range or to determine relative power level changes in an input signal. A typical system utilizing logarithmic amplifiers is an amplitude comparison, direction finding system wherein two receiving channels are utilized, the difference in signal level in each channel being related to the direction of an incoming signal with respect to a reference direction. The direction finding accuracy of this type of system depends upon both channels being closely matched so that amplitude distortion in one channel with respect to the other channel during the amplification process is minimized. One of the largest contributors to error budgets for these types of systems is in the logarithmic amplification portion of each channel. Thus, it is important that each logarithmic amplifier be calibrated with respect to the other logarithmic amplifier so that signal level differences can be accurately determined. Conventional systems have utilized several schemes to effect this calibration. One has been to simultaneously provide a plurality of calibration signals to each of the logarithmic amplifiers, each calibration signal corresponding to a predetermined signal level, and to measure the output signal of each amplifier. A difficulty with this scheme has been in developing precisely known calibration signals in order to obtain a calibration curve having closely spaced calibration points. For most applications, it is desirable to have at least six calibration points for each 10 db change in input signal level. This requires that calibration signals having 2 db increments be developed, these signals having a total dynamic range that could typically be as large as 70 db. The cost of providing these calibration signals to the accuracy required is high, and test equipment of this type must be calibrated frequently.
Another scheme utilized by conventional systems for calibrating logarithmic amplifiers utilizes an exponentially decaying calibration signal from a high Q resonator which drops a predetermined number of decibels in a predetermined time. This calibration scheme requires an accurate adjustment of the Q of the resonator developing the exponentially decaying calibration signal in order to obtain the predetermined decibel drop in the predetermined time. The adjustment usually requires the use of precision resistors and requires considerable time to make. The present invention solves both of the above problems by eliminating the need for developing a plurality of calibration signal each separated by a predetermined db increment, or the need for providing an exponentially decaying calibration signal which will drop by a predetermined number of decibels in a predetermined time.